ixc is an index specifying the potential model to use for the fine structure and the optional ixc0 is the index of the model to use for the background function.
The calculated potential can be corrected by adding a constant shift to the Fermi level given by vr0 and to a pure imaginary ``optical'' potential (i.e., uniform decay) given by vi0. Typical errors in FEFF's self-consistent Fermi level estimate are about 1 eV. (The CORRECTIONS card is similar but allows the user to make small changes in vi0 and vr0 after the rest of the calculation is completed, for example in a fitting process.)
The Hedin-Lundqvist self-energy is used by default and appears to be the best choice for most applications we have tested in detail. The partially nonlocal model (ixc=5) gives slightly better results in some cases, but has not been tested extensively.
Another useful exchange model is the Dirac-Hara exchange correlation potential with a specified imaginary potential vi0. This may be useful to correct the typical error in non-self-consistent estimates of the Fermi level of about +3 eV and to add final state and instrumental broadening.
Defaults if the EXCHANGE card is omitted are: ixc=0 (Hedin-Lundquist), vr0=0.0, vi0=0.0. For XANES, the ground state potential (ixc0=2) is used for the background function and for EXAFS the Hedin-Lundqvist (ixc0=0) is used.
Indices for the available exchange models:
*Hedin-Lundqvist -2eV edge shift and 1eV expt broadening EXCHANGE 0 2. 1. *Dirac-Hara exchange -3 eV edge shift and 5 eV optical potential EXCHANGE 1 3. 5.